Remove HAVE_SRTP define and unmaintained code.

It was defined unconditionally and the code for non-HAVE_SRTP was unmaintained
and failed to compile.

BUG=webrtc:7294

Review-Url: https://codereview.webrtc.org/2729373002
Cr-Original-Commit-Position: refs/heads/master@{#17074}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: eaa9c1db73133d771ea7a9b7ceb39b4286df0e78
7 files changed